(1) Field of the Invention
The present invention relates to a continuously variable transmission, which is particularly, but not exclusively, suited to use in motor vehicles including automobiles. More specifically, the invention relates to a multi-regime continuously variable transmission which has improved responsiveness during regime switching.
(2) Description of Related Art
WO 03/100295 (Torotrak (Development) Limited et al) discloses a continuously variable transmission which employs a toroidal race variator and in which various members are disposed coaxially. This continuously variable transmission provides forward and reverse gears suitable for a vehicle, including geared neutral, by combining the variator with a planetary gear device whose output speed is determined by both (a) the variator's output speed and (b) the speed of the transmission's input shaft. The transmission employs torque recirculation.
The variator of WO 03/100295 comprises: two input disks each having semi-toroidally recessed surfaces; one output disk which is positioned between the input disks, and each side surface of which has a semi-toroidally recessed surface; and rollers which make contact with the facing recessed surfaces of the input disks and the output disk. The input disk on the input side (engine side) is secured to an input shaft, while the output disk is linked to a sleeve forming a rotary output which is positioned around the input shaft. A boss part on one of the input disks is inserted into the sleeve, the input shaft and the input-disk boss being linked to the same element (a carrier) of a planetary gear device, and the sleeve shaft being linked to another element (a sun gear) of the planetary gear device.
WO 07/003657 (Torotrak (Development) Limited et al) discloses a continuously variable transmission in which an “end stop” function is provided, to prevent the variator from going beyond the end of its intended ratio range, by means of one-way clutches which prevent the speed of a first shaft, which drives the transmission output in low regime, from exceeding the speed of a second output shaft, which drives the transmission output in high regime.
Japanese patent application 2006-292079 describes a continuously variable transmission which comprises a full-toroidal continuously-variable transmission device (a “variator”), a planetary gear mechanism, a reverse-rotation gear mechanism, and a low/high switching mechanism. The planetary gear mechanism has a carrier coupled to an input disk of the full-toroidal continuously-variable gearing device and to an input shaft, a first sun gear coupled to an output disk of the full-toroidal continuously-variable gearing device, a rear carrier constituting an output element in a low regime, and a second sun gear constituting an output element in a high regime. In the low regime rotation of the rear carrier is transmitted via the reverse-rotation gear mechanism to the output shaft. In the high regime the rotation of the second sun gear is transmitted via a high clutch to the output shaft. In low regime power is recirculated through the planetary gear mechanism, whose output speed is determined both by the speed of the input disk and that of the output disk. The transmission has a low/high switching mechanism for switching between (a) the low regime, involving the engagement of a low clutch, in which rotary drive is transmitted via the planetary gear mechanism, with power recirculation, to the output shaft, and (b) the high regime, involving the engagement of a high clutch, in which the rotary output of the continuously-variable gearing device is transmitted to the output shaft. The transmission has a hydraulic actuator for controlling the movement of the rollers. A pressure difference applied to the hydraulic servo determines the force it applies. The hydraulic actuator acts upon a rotation and inclination support part, providing a reaction force opposing the force of traction occurring at the rollers. The direction of the reaction force output by the hydraulic actuator is switched whenever switching takes place between the low regime and the high regime.
The switching timing for the clutch and other elements is normally as follows for configurational reasons. FIG. 5(a) is a graph of transmission ratio during regime change in a continuously variable transmission of the type under discussion; (b) is a graph illustrating changes in the state of engagement (contact) and disengagement of the low clutch used in the low regime; (c) is a graph illustrating changes in the state of engagement and disengagement of the high clutch used in the high regime; and (d) is a graph illustrating switching of the direction of the reaction force (the direction of the servo pressure difference) output by the hydraulic actuator used in roller position control.
In a vehicle equipped with this continuously variable transmission, regime switching will take place if, for instance, kick-down occurs while in the high regime in which the high clutch is engaged. In such situations, the system (at time t11) starts engagement of the low clutch which had previously been disengaged, and, once the engagement of the low clutch has been completed (time t12), it switches the direction of the reaction force (the direction of the pressure difference) of the hydraulic actuator (e.g. from 1 (MPa) towards −1 (MPa), although the magnitude of the force will typically be changed as well as its direction. Once the switching of the reaction force direction has been completed (time t13), the system starts disengagement of the high clutch which had until then been engaged. The transmission ratio is fixed at the “synchronous” ratio (−0.3 in the present example) while both clutches are engaged. After release of the old clutch (t14) changing of the transmission ratio resumes.
Because the gearing ratio is fixed during the regime switching between times t11 and t14 it follows that, despite the requirement for rapid gear changing across regimes at times such as kick-down, sluggish gear changes are liable to occur. Hence there is a pressing demand for regime switching to proceed more rapidly in order to achieve a prompt response to driver demands.